Flowering Phenology As a Functional Trait in a Tallgrass Prairie

Research

Flowering phenology as a functional trait in a tallgrass prairie

Joseph M. Craine1, Elizabeth M. Wolkovich2, E. Gene Towne1 and Steven W. Kembel3 1Division of Biology, Kansas State University, Manhattan, KS 66502, USA; 2Ecology, Behavior & Evolution Section, University of California, San Diego, 9500 Gilman Drive #0116, La Jolla, CA 92093,USA; 3Center for Ecology & Evolutionary Biology, University of Oregon, Eugene, OR 97403, USA

Summary

Author for correspondence: • The timing of flowering is a critical component of the ecology of plants and has the poten- Joseph M. Craine tial to structure plant communities. Yet, we know little about how the timing of flowering Tel: +1 785 532 3062 relates to other functional traits, species abundance, and average environmental conditions. Email: [email protected] • Here, we assessed first flowering dates (FFDs) in a North American tallgrass prairie (Konza Received: 18 August 2011 Prairie) for 431 herbaceous species and compared them with a series of other functional traits, Accepted: 29 September 2011 environmental metrics, and species abundance across ecological contrasts. • The pattern of FFDs among the species of the Konza grassland was shaped by local climate, New Phytologist (2011) can be linked to resource use by species, and patterns of species abundance across the land- doi: 10.1111/j.1469-8137.2011.03953.x scape. Peak FFD for the community occurred when soils were typically both warm and wet, while relatively few species began flowering when soils tended to be the driest. Compared with late-flowering species, species that flowered early had lower leaf tissue density and were Key words: climate, community assembly, drought, grass, Konza Prairie. more abundant on uplands than lowlands. • Flowering phenology can contribute to the structuring of grassland communities, but was largely independent of most functional traits. Therefore, selection for flowering phenology may be independent of general resource strategies.

Introduction If the timing of flowering is an important component of community assembly, there should be general rules that pattern varia- The timing of flowering is a critical component of the ecology of tion in flowering for a community (Armbruster et al., 1994; plants and can be an important component of community assem- Morales et al., 2005; Elzinga et al., 2007). If flowering time is bly as flowering phenology influences not only the relative abun- neutral within a growing season and uninfluenced by species dance of species in a given ecosystem, but also their presence or interactions and environmental conditions within the growing absence (Rathcke & Lacey, 1985; Sargent & Ackerly, 2008; season then a null-model such as the mid-domain hypothesis Crimmins et al., 2011). One manner in which flowering phenol- (Morales et al., 2005) may accurately predict the pattern of flow- ogy affects the composition of plant communities is through its ering phenology at the community level. The mid-domain effect on species interactions. Overlap of flowering times among hypothesis predicts that flowering for a flora should be greatest in species is almost inevitable in most communities, generating the the middle of the growing season as a consequence of the potential for strong competition, but also facilitation, for pollina- constraints of species placement in a bounded growing season tion resources (Rathcke & Lacey, 1985). Independent of pollina- (Morales et al., 2005). If flowering time, however, is critical to tion, flowering phenology indirectly affects species interactions, competition for pollinators and other resources (Kochmer & as flowering times may be associated with other aspects of perfor- Handel, 1986; Rathcke, 1988; Godoy et al., 2009) or avoidance mance, such as canopy development (Cleland et al., 2006), that of environmental stress (Reich & Borchert, 1984; Penuelas et al., influence competition for resources required for vegetative 2004), patterns may deviate from null model expectations. For growth. While many temperate woody species flower before example, in many grasslands, soil moisture stress can strongly leaves are produced, most temperate grassland species flower at limit plant production and reproduction. Even in grasslands the time of maximum vegetative biomass (Mooney et al., 1986; where growing season length is dictated by temperatures, soil Sun & Frelich, 2011), causing species with similar flowering moisture stress can be high in the middle of the growing season times to also compete for limiting soil resources and ⁄ or light. as leaf area develops, temperatures peak, precipitation declines, Flowering phenology can also influence plant success indepen- and soil moisture is depleted (Briggs & Knapp, 1995; Nippert dently of species interactions. Flowering when environmental et al., 2006; Craine et al., 2010). In such grasslands, midsummer stress is typically high, such as when temperatures are frequently soil moisture stress in many grasslands is frequent even in years cold or soil moisture is typically low, can also lower plant fecun- with above-average precipitation (Nippert et al., 2011). dity if not lead to the species’ local extirpation (Lacey et al., If flowering time for a species influences its exposure to envi- 2003; Inouye, 2008). ronmentally stressful conditions and its interspecific interactions

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(Brody, 1997; Augspurger, 2009; Devaux & Lande, 2010) then Flowering should be reduced during times of consistent extreme it could also be part of a larger plant strategy that incorporates temperatures and ⁄ or minimum soil moisture. We test this other functional traits. Flower production and maintenance as hypothesis by comparing the frequency of first flowering dates well as subsequent reproduction is resource-intensive (Ashman & (FFDs) across species with long-term records of air temperatures, Schoen, 1994; Obeso, 2002), which might tend to favor repro- precipitation, and soil moisture. Secondly, if environmental stress duction during times of low environmental stress. For example, is a determinant of flowering for a given species, species that high leaf tissue density is favored under conditions of low nutri- flower during the most stressful times should have traits that con- ent availability in grasslands and considered part of a broader fer resistance to environmental stress or low resource availability. low-nutrient strategy (Craine et al., 2001; Craine, 2009; Craine To test this hypothesis, we examined relationships between & Towne, 2010). Thus if nutrient availability varies seasonally, flowering time and average leaf tissue density – a proxy for stress species that flower during periods of low nutrient availability tolerance – for c. 400 species, as well as relationships over a might also have high leaf tissue density. Seasonal variation in smaller number of species with other functional traits such as drought could also generate links between plant functional traits physiological drought tolerance and photosynthetic rates that are and flowering phenology. For example, when stresses become associated with plant resource strategies. severe enough to constitute a disturbance (e.g. prolonged Lastly, to determine whether the timing of flowering contrib- drought), species with high-activity, rapid-turnover tissues would utes to the abundance of a species, we examined relationships be favored. For example, severe droughts that generate wide- between flowering time and abundance across three main ecolog- spread mortality across all species can favor annuals over perenni- ical contrasts at Konza: topography, fire, and grazing. At Konza als (Weaver, 1968). If the probability of drought varies Prairie, uplands have shallower soils than lowlands and species seasonally, times of typical low soil moisture could be more likely that are more physiologically drought-tolerant (Craine et al., to have species with traits that confer drought resistance, while 2011). We compared watersheds that are burned annually in the times after the drought could be associated with species that have spring with those that are infrequently burned because annual traits associated with disturbance, such as low leaf tissue density. spring burning might preferentially decrease the abundance of Ultimately, if flowering phenology is not neutral and is associ- early flowering species (Howe, 1994). Finally, we compared the ated with responses to environmental conditions and plant abundance of species with different flowering times in areas functional traits then flowering phenology should also be linked grazed by bison with those not grazed by bison. Although grazing to species abundance on a landscape (Willis et al., 2010). For decreases the abundance of C4 grasses and increases species rich- example, if environmental stress varies over the course of a grow- ness (Knapp et al., 1999; Collins & Smith, 2006), we do not ing season, species that flower during stressful times might be less know, for example, whether grazing increases the richness of spe- abundant, linking flowering time and abundance. Competition cies that flower late in the growing season as a consequence of for pollinator resources could in turn strengthen associations competitive release. between abundance and flowering time driven also by patterns of environmental stress. If competition for pollination resources is Materials and Methods an important associate of flowering time, then there might be fewer species that flower during the same time as the most The study was conducted at Konza Prairie Biological Station, a abundant species to minimize competition. This would lead to 3487 ha native tallgrass prairie located in northeastern Kansas, fewer less-abundant species flowering at the time when more- USA (39.08N, 96.56W) (Knapp et al., 1998). Mean annual abundant species flower. temperature is 13C, with average monthly temperatures ranging To test for evidence that flowering time is important to com- from )3C in January to 27C in July. Annual precipitation for petition for resources and avoidance of environmental stress, we Konza Prairie averaged 844 mm from 1983 and 2009, with c. compared interspecific patterns of the timing of flowering of a 75% falling in the April–September growing season and peak North American tallgrass prairie (Konza Prairie) with a suite of precipitation in June. The vegetation at Konza is primarily unp- functional traits, environmental data, and species abundance lowed native tallgrass prairie. Woody species form gallery forests across three ecological contrasts. We assessed dates of first flower- in riparian areas, and can be abundant in specific watersheds, ing for 430 herbaceous species with near daily surveys of the vege- depending upon fire frequency (Briggs et al., 2002). The known tation. Patterns of flowering phenology were then determined for vascular flora of Konza Prairie comprises 597 species, of which the Konza flora as well as for groups defined by species’ photo- 59 are woody. Of the 539 herbaceous species, 122 are grami- synthetic pathway, life history, and whether the species were noids, 411 are eudicots, and six are ferns. Graminoid species con- native or not to North America. sist of C4 Poaceae (51 species), C3 Poaceae (38 species), C3 To better understand the interplay of flowering phenology Cyperaceae (26 species), and C4 Cyperaceae (seven species). Of with environmental conditions, functional traits, and abundance, the herbaceous eudicots, 397 species utilize the C3 photosyn- we focus on three main hypotheses. First, if the probability of thetic pathway and 14 have the C4 photosynthetic pathway. species flowering over the growing season is influenced by envi- At Konza, grazing, burning, and landscape position are the ronmental stress, then the greatest number of species should main environmental contrasts that affect plant communities begin flowering when growing conditions are the least stressful, other than climate. Grazing on three watersheds by reintroduced which would be when soils are the warmest and wettest. native bison began in October 1987 and expanded to another

New Phytologist (2011) 2011 The Authors www.newphytologist.com New Phytologist 2011 New Phytologist Trust New Phytologist Research 3 three watersheds in 1992. Stocking rate increased over time so diptest in R 2.12.0). Patterns of FFD across the growing season that grazing intensity removes c. 25% of the grass production were compared for physiotaxonomic functional groups (C3 and (Towne, 1999). Compared with areas without bison, grazed areas C4 Cyperaceae, Poaceae, and eudicot species) as well as for spe- are more diverse (Collins & Smith, 2006), have higher abun- cies native and not native to North America and across three lon- dance of forbs (Towne et al., 2005), and greater nutrient avail- gevity categories (annual, biennial, or perennial). To compare ability (Johnson & Matchett, 2001; Veen et al., 2008). Annually categories, means were compared with ANOVA while the tim- burned watersheds tend to have lower nutrient availability (Blair, ings of peak FFD were compared nonstatistically (i.e. visually) on 1997) than infrequently burned watersheds. Lastly, upland and smoothed curves. lowland positions differ mainly in soil depth. Upland soils are To determine whether peaks and troughs in flowering were shallow (often < 25 cm) (Schimel et al., 1991) and are generally associated with climate parameters, patterns of FFD were com- cherty, silty clay loams overlying limestone and shale layers (Udic pared with mean daily temperature and mean daily precipitation Argiustolls, Florence series). By contrast, lowland soils are deeper collected from Konza and averaged over 25 yr. In addition to and derived from colluvial and alluvial deposits (Pachic Argius- temperature and precipitation, patterns of FFD were compared tolls, Tully series). Lowland soils are less xeric (Schimel et al., with soil moisture which was measured biweekly during the 1991; Nippert & Knapp, 2007b) and support greater productiv- growing season for 25 yr (1984–2008) at two points in the low- ity and flowering (Heisler & Knapp, 2008; Craine et al., 2010). lands of an annually burned ungrazed watershed. Soil moisture Herbaceous species on Konza were surveyed for first flower was measured with a neutron depth moisture gauge (Troxler appearance almost daily from March to October 2010 (129 d Electronic Incorporated, Research Triangle Park, NC, USA) in sampled over 189 d period), throughout Konza. When a species thin-walled aluminum access tubes buried 2 m deep. Data on soil was found to be flowering for the first time, the date was recorded moisture at 25 cm are used here as they likely are most important and the plant collected. Occasionally, some outlier individuals for water relations of the majority of species (Nippert & Knapp, flowered well before the majority of other individuals in their 2007a; Craine et al., 2010). Soil moisture data were expressed as population but these nonrepresentative samples were not col- an index of apparent field capacity (Craine et al., 2010). lected. We assessed FFDs for a total of 430 herbaceous species In addition to the leaf traits measured on most species, FFDs (Table S1). were compared with seven additional functional traits previously For most species that were encountered in a flowering state, a measured on plants grown under controlled conditions (Tucker series of leaf traits were measured on nonsenescent leaves of a et al., 2011). These functional traits relate to resource capture range of ages. Some species flower with few leaves present (e.g. and retention and are those likely to represent plant resource Spiranthes vernalis), while others have too highly dissected leaves strategies, such as adaptations to low water, light, or nutrient to measure accurately (e.g. Lomatium foeniculaceum). Conse- availability or to high-resource environments (Craine, 2009). quently, leaf traits for these species were not measured. Thus we Functional traits included maximum photosynthetic rates, maxi- were able to measure leaf traits on a range of ages of nonsenescent mum stomatal conductance, physiological drought tolerance, leaf leaves for 391 species. After collection in the field, plants were angle, average fine root diameter and tissue density, and fraction stored in a plastic bag with a small amount of water until leaf area of biomass in roots. Functional trait data existed for 86–93 of the could be measured on a LI-3100 leaf area meter (Li-Cor, Lincoln, species measured here, depending on the trait. Seed mass data for NE, USA) later that day. Thickness was measured with calipers 335 species were acquired from the Seed Information Database on typically three to five leaves on an area adjacent to any midrib of the Royal Botanic Gardens, Kew (http://data.kew.org/sid). In or major secondary veins. Leaves were then dried at 65C for addition to the traits examined here, we analyzed the relation- 2–3 d and weighed. Leaf volume was calculated as the product of ships between flowering phenology and species’ geographic leaf area and thickness, and leaf tissue density was calculated as climate envelopes, flowering color, phylogenetic patterns, and the ratio of dry mass to volume. foliar C and N isotope ratios from leaves collected in the field. Dried leaf matter was then ground and analyzed on a Delta Details on the methods and results for these metrics are presented Plus mass spectrometer (ThermoScientific, Bremen, Germany) in in Notes S1. combination with a CE 1110 elemental analyzer (Carlo Erba, Lastly, to better understand whether FFDs were related to Milan, Italy) for C and N concentrations as well as C and N iso- abundance of species, we compared FFDs with the abundance of tope ratios (see Supporting Information, Notes S1). species across a number of ecological contrasts (Craine & Towne, 2010): landscape position (upland and lowland), grazing by native bison (grazed and ungrazed), and burning (frequent, Statistical analyses annual burning vs infrequent, c. every 20 yr). Species abundance To assess whether community-level phenology patterns deviated was determined on 16 watersheds from 1994 to 2009. The fire from a simple mid-domain neutral model, we first smoothed uni- treatments for most watersheds have been in place since 1983, variate community-level patterns with a kernel density smoothing except for four watersheds which had their fire treatments (annu- function (h = 12) and tested for a significant departure from ally burned and unburned) reversed in 2001. These four water- unimodality in community-level flowering phenology using sheds were classified by their current treatment. In each Hartigan’s dip test (Hartigan & Hartigan, 1985) with signifi- watershed, 40 plots for determining species abundance are cance tested using 999 reps of a Monte-Carlo approach (package divided evenly between the shallow xeric upland soils and the

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mesic lowland soils. In addition to examining relationships with and C3 eudicots (June 16 ± 2 d). Although C4 Poaceae (July average abundance, relationships were also examined with the 15 ± 6 d) and C4 eudicots (July 24 ± 15 d) flowered, on average, differences in log-transformed abundance between uplands and distinctly later than the C3 functional groups, there was little dif- lowlands, frequently and infrequently burned, and grazed and ference in the timing of flowering for the first or last species of C3 ungrazed. The latter two metrics can be considered the species’ and C4 Poaceae. For example, the first C3 Poaceae species to long-term responses to fire and grazing. flower was Poa pratensis on April 21, whereas the first C4 Poaceae species to flower was Bouteloua dactyloides on April 27, which was Results before all C3 grasses except P. pratensis. Tripsacum dactyloides, another C4 grass, also flowered early in the season, with the FFD on April 28. Conversely, C Poaceae species flowered, on average, General patterns and differences among groups 3 before the C4 Poaceae species, but some C3 Poaceae species began Among the 430 herbaceous species observed in flower at Konza, to flower when most C4 Poaceae were flowering. For example, the first species to flower was the annual forb Holosteum umbella- the FFDs for Leersia virginica and Leersia oryzoides were July 23 tum on March 30. The last species to begin flowering was the and September 9, respectively. Further reducing evidence for the perennial forb Gentiana puberulenta on October 5. The total role of photosynthetic pathway in timing of flowering, C3 eudi- range of FFDs was 189 d. Based on smoothed curves, FFDs cots flowered throughout the entire growing season (see earlier peaked on June 14 with 0.83% of the flora (3.6 species) flowering discussion). on that day (Fig. 1a). There was a secondary peak on August 21, The nonnative species that have become established at Konza with 0.38% of the flora flowering, that followed an intermediate characteristically flower in the first half of the growing season trough on August 8 (Hartigan’s dip test, P = 0.02). (Fig. 1d,e). Species not native to North America flowered on On average, species with the C3 photosynthetic pathway flow- average 21 d earlier than native species (May 31 ± 5 d vs June ered 33 d earlier than species with the C4 photosynthetic path- 21 ± 2 d, P < 0.001). The early average FFD for nonnative spe- way (June 13 ± 2 vs July 16 ± 6, P < 0.001) (Fig. 1b,c). Among cies was associated with a near absence of nonnative species flow- the five functional groups, FFD was earliest for the C3 Cypera- ering late in the season. Only four nonnative species flowered ceae (mean FFD, May 10 ± 10 d), C3 Poaceae (May 24 ± 8 d) after August 1 (6% of the nonnative community) as opposed to

(a)

) 0.01 4 ) –1 –1 0.008 3 0.006 2 0.004 0.002 1 Flowering rate (S d rate Flowering Fraction flowering (d flowering Fraction 0 0 75 125 175 225 275 Day of year (b) (c)

) 0.02 3 ) –1 C3P –1 2.5 0.016 C D C P 3 C C 4 C D 2 0.012 3 4 1.5 0.008 1 C P C3P 4 C D 0.004 0.5 C3C 4

C D (S d rate Flowering Fraction flowering (d flowering Fraction 0 3 0 (d) (e)

) 0.01 3 ) –1

Non-native –1 2.5 0.0075 Native Native 2 0.005 1.5 1 Non-native 0.0025 0.5 Flowering rate (S d rate Flowering Fraction flowering (d flowering Fraction 0 0 (f) (g) )

) 0.01 3 –1

–1 Perennial Biennial 2.5 Perennial Fig. 1 (a) Patterns of first flowering dates (FFDs) for 0.0075 Annual 2 Konza herbaceous flora expressed as a fraction of all 0.005 1.5 recorded species, flowering per d or number of species (S) Annual –1 1 flowering per d (S d ). Also shown are FFDs compared 0.0025 0.5 between: (b, c) functional groups (C3 Cyperaceae, C3 Flowering rate (S d rate Flowering Biennial eudicots, C euDicots, C Poaceae, and C Poaceae); Fraction flowering (d flowering Fraction 0 0 4 3 4 75 125 175 225 275 75 125 175 225 275 (d, e) native and nonnative species; and (f, g) life history Day of year Day of year groups (annual, biennial, and perennial species).

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0.009 0.9 4.5 of maximum soil moisture by August 7, which corresponded 0.008 4 with the August 8 FFD trough. Peak first flowering was also coin- 0.8 0.007 3.5 ) ) cident with peak precipitation. The mean temperature on the day –1 0.006 0.7 3 –1 the first species flowered was 9.3C, while the mean temperature 0.005 2.5 on the day the last species began flowering was 15.8C. 0.6 0.004 2

0.003 0.5 1.5 Relative soil moisture Relative

Precipitation (mm d Relationships with functional traits Fraction flowering (d flowering Fraction 0.002 1 0.4 0.001 0.5 Among the leaf traits measured on plants in the field, species that 2 0 0.3 0 flowered later had relatively high leaf tissue density (r = 0.07, 75 125 175 225 275 Day of year P < 0.001) (Fig. 3). Based on the relationship with flowering Fig. 2 Fraction of Konza flora flowering each day (black line with embed- time, the leaf tissue density of the last species flowering would be ded circles). Each circle represents a day where a new species in flower was 68% greater than the first species flowering. Species that flowered observed. Also shown are an index of soil moisture (thick gray line) and later had marginally thinner leaves (r2 = 0.01, P = 0.02), lower average daily precipitation (thin gray line). 2 2 [NL](r = 0.05, P < 0.001), and higher [CL](r = 0.02, P = 0.008) than early-flowering species (Fig. 3). Based on these ) 73 native species (19% of the native community). Among species relationships, the last flowering species would be 10.9 mg N g 1 ) that flowered before August 1, 27% of the nonnative community lower and 19.8 mg C g 1 higher than the first flowering species. and 18% of the native community flowered before May 1. Aver- When flowering times were compared with ecophysiological age flowering times among annuals, biennials, and perennial spe- traits measured under common conditions, FFD was not related cies were not significantly different (P = 0.68) (Fig. 1f,g). The to photosynthetic rate, stomatal conductance, physiological earlier flowering of nonnative species could not be ascribed to a drought tolerance, leaf angle, root diameter, root tissue density, 2 greater proportion of C3 species being nonnative than native. or root fraction (r < 0.04, P>0.05 for all). Relationships were There was no difference in the proportion of native and non- not significant even when controlling for average differences native species that are C3 and C4 (P = 0.57). between the two phylogenetic clades (monocots and eudicots; Both peak FFDs and the early August trough in FFDs corre- data not shown). Species with small seeds did not flower earlier sponded to mean climate parameters (Fig. 2). Across the years, or later than species with larger seeds. There was no relationship soil moisture averaged 83% of maximum on March 30, and then between log-transformed seed mass and flowering time declined throughout the growing season, reaching a low of 47% (P = 0.73, n = 335).

(a)0.8 (b) 70

0.7 60 0.6 50

0.5 –1 40 0.4 ] (mg g ) ] (mg L 30 0.3 [N 20 Leaf thickness (mm) Leaf thickness 0.2

0.1 10

0 0

50 –3 0.8

) 0.7 –1 45 0.6 0.5 ] (mg g L

[C 40 0.4 Fig. 3 Bivariate regressions between ﬁrst ﬂowering date 0.3 and leaf thickness (y = 0.26 ) 0.00030x, r2 = 0.01,

35 Leaf tissue density (g cm 0.2 P = 0.02) (a); foliar N concentrations ([NL]; y = 33.7 ) 0.052x, r2 = 0.04, P < 0.001, n = 365) (b); foliar C 0.1 2 concentrations ([CL]; y = 40.4 + 0.011x, r = 0.02, 30 0 P = 0.003, n = 365) (c); and leaf tissue density (y =0.19+ 75 125 175 225 275 75 125 175 225 275 0.00097x, r2 = 0.07, P < 0.001, n = 392) (d). Day of year Day of year

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(a) 2 (b) 4

1 3

0 2

–1 1

–2 0 Abundance –3 –1 Diff abund (upl-lowl) Diff abund –4 –2

–5 –3

(c) 5 (d) 3 4 2 Fig. 4 (a) Relationship between log-transformed 3 abundance and ﬁrst ﬂowering dates (FFDs). Also shown 1 2 are relationships between FFDs and the difference in log-transformed abundance between uplands and low- 1 0 lands (‘diff abund (upl-lowl)’) (y = 0.82 ) 0.0058x, 2 0 r = 0.05, P = 0.002, n = 170) (b); grazed and ungrazed –1 watersheds (‘diff abund (graz-ungraz)’) (c); and burned –1 and unburned watersheds (‘diff abund (burn-unburn)’)

Diff abund (graz-ungraz) Diff abund –2 –2 (burn-unburn) Diff abund (d). A value of 1 for differential abundance represents a species that is 10 times more abundant in uplands than in –3 –3 lowlands, whereas a value of )1 represents a species that 75 125 175 225 275 75 125 175 225 275 is 10 times more abundant in lowlands than in uplands. Day of year Day of year Dashed lines are not signiﬁcant at P < 0.05.

watersheds, and the same was true for their abundance in un- Relationships with abundance grazed watersheds (P>0.1 for both). Likewise, species that Across all contrasts, species that flowered earlier were not more or increased in abundance with grazing did not flower earlier or later less abundant than species that flowered later (P = 0.08; Fig. 4a). than species that declined with grazing (Fig. 4c, P = 0.16). Flow- Species that were more abundant in uplands did not flower ear- ering phenology was not associated with the relative abundance lier or later than species that were less abundant in uplands of species in frequently burned (P = 0.15) or infrequently burned (P = 0.71). Likewise, species that were more abundant in low- regions (P = 0.39), or with species’ response to burning (Fig. 4d; lands did not flower earlier or later than species that were less P = 0.73). abundant in lowlands (P = 0.19). Yet for species found in both uplands and lowlands, those that were differentially more abun- Discussion dant in uplands than in lowlands flowered earlier than those that were more abundant in lowlands than in uplands (r2 = 0.05, Environmental stress and climatic constraints on flowering P = 0.004, n = 168; Fig. 4b). Based on the relationship between FFD and the differential abundance of species in uplands and Understanding the role of plant phenology in structuring plant lowlands, for every 52 d earlier that a species flowered, it was two types and communities has long been an active research focus times more abundant in uplands than in lowlands. Part of the (Parrish & Bazzaz, 1979; Ollerton & Lack, 1992; Parmesan & reason that the difference in abundance between uplands and Yohe, 2003; Menzel et al., 2006), but this has resulted in little lowlands was significant but the relationships with abundance consensus about its role in ecological communities. Climate were not significant was that species found in lowlands but not in shapes Konza flowering patterns both by setting the seasonal uplands tended to flower later in the growing season and were bounds for flowering and by shifting stressors during the growing found at low abundance. When these species were excluded from season. Low temperatures that define the start and end of the analyses, species that flowered later were significantly more abun- growing season place fundamental limits on the period of plant dant in the lowlands than species that flowered earlier (r2 = 0.05, flowering in many temperate systems (Larcher, 2003; Schwartz, P = 0.002, n = 170). There was still no relationship between 2003; Inouye, 2008). flowering time and abundance in the uplands when including Although the community flowering curve of Konza may be only species found in both uplands and lowlands. characterized as a neutral model with multiple bounds through- Species’ flowering phenology was not associated with the spe- out the growing season, more parsimoniously, flowering phenol- cies’ responses in abundance to grazing or burning. Species that ogy is an important mechanism to avoid environmental stress were more abundant in grazed watersheds did not begin flower- within the growing season, which structures plant assemblages ing earlier or later than species that were less abundant in grazed and diversity patterns across Konza’s grassland habitats. Although

New Phytologist (2011) 2011 The Authors www.newphytologist.com New Phytologist 2011 New Phytologist Trust New Phytologist Research 7 the hump-shaped seasonal pattern of community flowering underlie it. Although we did not see a decline in foliar d15N broadly fits a bounded neutral model such as the mid-domain (Fig. S3) that would suggest lower N availability later in the (Morales et al., 2005), the flowering patterns are as parsimoni- season (Craine et al., 2009), foliar N concentrations of late- ously, if not more so, explained by species flowering tracking flowering species were lower, consistent with the general decline patterns of within-season environmental stress. Further evidence in N concentrations over time seen for individual species (Rao supporting community-level flowering being nonneutral at et al., 1973) and the seasonal decline in soil N mineralization Konza is the late-season trough in flowering that corresponds to a and soil respiration previously observed (Turner et al., 1997; seasonal maximum in soil moisture stress. Although specific Johnson & Matchett, 2001). With few linkages between flower- patterns in soil moisture vary across years, the long-term effects ing phenology and functional traits, more research is necessary to of repeated drying during this period would likely select against understand the linkages that are present and to search for species that begin flowering during a period of typically high other functional traits that might be associated with flowering environmental stress. The species that do begin flowering during phenology. this period are a mix, including many species that are likely to Outside of the associations with resource-related traits, most occupy sites that would be least impacted by drought or have other traits studied were orthogonal to flowering. Flowering other strategies to avoid it. For example, many of the species that timing was distributed relatively evenly across species with differ- first begin to flower during this period are summer annuals, for ent flower colors (Fig. S1). Annuals did not necessarily flower example, Ambrosia artemisifolia, and would occupy disturbed earlier or later than perennials, most likely because of the presence sites where competition for water might be low, even in a dry of both winter and summer annuals at Konza (Towne, 2002). year. Other species that flower during this period, such as Lobelia Furthermore, although C3 grasses on average flowered earlier than cardinalis, Lobelia siphilitica, and Amaranthus tuberculatus, are C4 grasses, photosynthetic pathway was not diagnostic for flower- considered wetland species. Still others such as Desmodium ing time. C4 grasses such as B. dactyloides and T. dactyloides sessilifolium and Helianthus maximiliani are deeply rooted and flowered before almost all of the C3 grasses, while there were C3 would be able to tap deeper soil water (Weaver, 1968). grasses and C3 eudicots that flowered after most C4 grasses had begun flowering. Thus, while the generality of early-flowering C3 and late-flowering C grass species was maintained on average, Resource strategies and functional groups 4 both groups had species that suggested phenological divergence Flowering time is expected to be a highly important niche trait from the general trend might be common. Similarly, while there (Donohue, 2005; Fargione & Tilman, 2005; Godoy et al., 2009; was nonrandom phylogenetic signal in flowering time (Fig. S2), Johnson, 2010). Yet, we found relatively few links between most and some taxa exhibited characteristic seasonal patterns of flower- functional traits and flowering time. If water and nutrient avail- ing, there was a great deal of variation in flowering time within ability decline as the growing season progresses, flowering phe- broader clades (Wilczek et al., 2009, 2010). nology has the potential to covary with functional traits that are Taken together, our results suggest that flowering phenology associated with resource availability. Along these lines, Konza does not fit into a general resource strategy in any direct way. Pre- species that flowered later in the growing season had higher leaf vious work suggests important functional traits should covary tissue density. Leaf tissue density has been shown to be associated with flowering time (Golluscio & Sala, 1993; Thuiller et al., with species that perform well when nutrients are limiting 2004); however, others have suggested phenology as a plant trait (Craine et al., 2001) as well as species that are physiologically tol- may be often neutral (Ollerton & Lack, 1992) and thus correla- erant of drought (Tucker et al., 2011). As such, it is possible that tions with resource use or related traits may be rare, as has been as the season progresses, nutrient and water availability decline found in other recent analyses (Willis et al., 2010) where correla- such that late-flowering species would benefit from having high tions with a variety of traits were uncommon. Overall, general leaf tissue density. direct tests such as those presented here are sorely needed across a Although soil moisture declines at Konza as the growing season broader array of species and regions before a general framework progresses and species water-use efficiency has been shown to can emerge. increase over time as soils dry out in other ecosystems (Smedley et al., 1991), there was no relationship between FFDs and Spatial and temporal patterns of abundance physiological drought tolerance. Examining patterns of carbon isotopes in leaves, late-flowering species at Konza actually showed Increasingly, researchers have speculated about (Augspurger, lower, not higher, water-use efficiency than early-flowering spe- 2009; Crimmins et al., 2010) and sometimes documented cies (Supporting Information Fig. S3). This is unlikely to be the (Chapin et al., 1995; Inouye, 2008; Willis et al., 2010) how result of increasing water availability for a given location over this shifts in phenology may be associated with changes in abundance. time, but instead likely represents late-flowering species occupy- However, we found no simple links between flowering time and ing wetter sites on average. As such, there was no support for the abundance. Community flowering appeared to vary strongly with idea that the high leaf tissue density of late-flowering species was environmental stress, for example, low temperatures or low soil associated with an increased ability to endure water stress. If moisture. Yet, species that flowered during periods of higher soil water stress was not driving the increase in leaf tissue density over moisture stress were as abundant as those that flowered at other time, then it is likely that declining nutrient availability might times. Further, flowering during the peaks of community-wide

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flowering, or later in the season when plant biomass is highest Additionally, if the mid-growing season drought becomes more (Schimel et al., 1991), did not correspond to species’ abundance. pronounced, this might reduce the number of species flowering Furthermore, we found no relationship between disturbance – during the early-August trough in flowering. Evidence of a such a via either grazing or fire – and flowering phenology. Management shift towards a novel midseason gap (or decrease) in flowering regimes in some systems can alter community-level phenology has already been suggested in other floras observationally via selection pressure (Ollerton & Lack, 1992), accelerating, for (Aldridge et al., 2011) and via experiments (Sherry et al., 2007). example, the flowering of species in habitats with consistent late- That said, our approach did not quantify intraspecific variation season cutting regimes (Lack, 1982). In Konza, however, flower- in flowering in response to changes in environmental factors. Bet- ing dates were equally variable across species more abundant in ter predictive capacity will require an approach that scales from grazed or burned plots vs control plots. The lack of linkage intraspecific to community levels and contrasts patterns across a between species’ flowering phenology and abundance response to landscape for each species. A plant’s phenology also extends grazing – which is continuous throughout the year – might not beyond the timing of first flowering to include peak flowering be as surprising as the lack of effect for the seasonally constrained and other reproductive events, as well as vegetative flowering such fires. Most burning occurs in March–April, as many species begin as the phenology of leaf production and stem elongation. At this flowering. However, species with flowering times during this per- point, more research is necessary to understand how coordinated iod do not tend to be less abundant in burned areas – suggesting the different phenological components of plant growth are across that the co-occurrence of flowering and disturbance does not species, which may aid in understanding broader plant strategies penalize overall abundance, at least in the presence of grazers. and patterns of plant performance (Wilsey et al., 2011). This could suggest that flowering time is an effectively neutral trait for these species (Ollerton & Lack, 1992) or effects may be Acknowledgements more nuanced. For example, early-season species tend to have higher variability in their FFDs across space and time (Menzel J.M.C was supported by a National Science Foundation (NSF) et al., 2006), and therefore early-season species at Konza may grant (DEB-0816629). This work was conducted while E.M.W vary their flowering between burned and unburned areas, delay- was a National Science Foundation Postdoctoral Research Fellow ing flowering in burned areas. Understanding the full effects of in Biology (DBI-0905806). The Konza Prairie LTER dataset fire on phenology would thus require landscape-level studies analyzed is the plant cover dataset (PVC02), soil moisture focused on intraspecific variation in flowering. (ASM01) and the climate data (ATP01). Data collection and First flowering time was, however, associated with the topo- archiving were supported by NSF grants to the Konza Prairie graphical patterning of abundance on the landscape. Konza is LTER program. Seed data were acquired from the Seed Informa- characterized by a heterogeneous landscape of cooler, moister tion Database (SID) of Royal Botanic Gardens, Kew, by W. lowland sites and hotter, drier upland sites (Nippert et al., 2011). Wen. The authors appreciate the comments of the editor and Many species show differential abundance between these two three anonymous reviewers. dominant microhabitats. Species with higher abundance on upland sites tended to flower earlier than those more common on References lowland sites. Such an effect could be driven by climatic differences: species that do best on upland sites are those adapted to Aldridge G, Inouye DW, Forrest JRK, Barr WA, Miller-Rushing AJ. 2011. flower before upland sites rapidly dry out, while, conversely, low- Emergence of a mid-season period of low floral resources in a montane meadow ecosystem associated with climate change. Journal of Ecology 99: land species may flower slightly later as a result of cooler lowland 905–913. temperatures. Thus, within Konza, the interaction of flowering Armbruster WS, Edwards ME, Debevec EM. 1994. 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